Vehicles such as wheel type loaders include work implements capable of being moved through a number of positions during a work cycle. Such implements typically include buckets, forks, and other material handling apparatus. The typical work cycle associated with a bucket includes positioning the bucket and associated lift arm in a digging position for filling the bucket with material, a rackback position, a raised position, and a dumping position for removing material from the bucket.
Control levers are mounted at the operator's station and are connected to a hydraulic circuit for moving the bucket and/or lift arms. The operator must manually move the control levers to open and close hydraulic valves which in turn cause the implement to move. For example, if the lift arms are to be raised, the operator moves the control lever associated with the lift arm hydraulic circuit to a position at which a hydraulic valve causes pressurized fluid to flow to the rod end of a lift cylinder thus causing the lift arms to rise. The operator must manually hold the hydraulic valve open with the control lever until the lift arms reach the desired height. When the operator returns the control lever to a neutral position, the hydraulic valve closes and pressurized fluid no longer flows to the lift cylinder.
To perform a work cycle in the most efficient manner, the operator must steer the vehicle while simultaneously operating both the lift arm control lever and the bucket control lever. Such operation can be tiresome and difficult, particularly for inexperienced operators.
However, since the implement is moved through the same positions each time the work cycle is repeated, control systems can be included to automate some of the work cycle functions thus reducing the operator's work load. Such control systems often provide predetermined positions for the vehicle implements which are associated with the above listed work cycle functions. When the operator moves a control lever beyond a preselected position at which the associated hydraulic valve is open, detents hold the control lever in that position. The detent continues to hold the control lever in this position until the lift arm or bucket reaches the predetermined position. When the control system senses that the implement has reached the predetermined position, the control system releases the detent. Since the control levers are typically spring biased towards a neutral position at which the hydraulic valve is closed, the control lever returns to the neutral position and the movement of the lift arm or bucket is stopped when the detent is released. Once the control lever is moved beyond the predetermined position, the control system allows the operator to release the control lever and concentrate on the next function to be performed.
Many such devices have been developed. One such device is described in U.S. Pat. No. 4,141,258, issued to Walzer on Feb. 27, 1979. Walzer discloses a mechanical detent system which includes a roller that is spring biased toward an engaged position with respect to a notch on the control lever. A solenoid is provided for controllably moving the roller in a direction away from the engaged position and a lost-motion slot is included to allow the operator to manually release the control lever from the detent position. While this arrangement adequately performs the necessary operations, it includes a large number of moving parts which increase maintenance and warehousing space.
Another detent mechanism is described in U.S. Pat. No. 3,915,325 issued to Lark et al. on Oct. 28, 1975. This mechanism utilizes electromagnets to hold the control lever in the desired position. Potentiometers are connected to the implement and to a mechanism for setting the desired implement position. When the potentiometers are producing substantially different signals, the electromagnet is energized. If the control lever is moved such that a latching means is relatively close to the electromagnet, then the electromagnet will attract the latching means with sufficient force to hold the control lever in that position. When the implement reaches the desired position, the two potentiometers produce nearly identical signals which causes the electromagnet to be deenergized. This in turn causes the control lever to return to the neutral position. While this arrangement reduces the number of moving parts, the electromagnets are energized whenever the implement is not in the desired position. For example, the detent mechanism is activated in situations in which the operator only moves the control lever a small amount and has no desire to put the control lever into the detent position. Furthermore, if the implement is above the preselected lifted position and the operator moves the control lever to further raise the implement, the electromagnet will also be energized. This results in an undue waste of electrical energy and does not accurately reflect the operator's desired operation.
The present invention is directed to overcoming one or more of the problems set forth above.